In the field of power conversion technology, increasing the conversion efficiency and improve the reliability are the most important demands. Basically, conventional buck converter technology is desirable as it transfers the power from input to output with controlled function. However, the efficiency of conventional buck converter is limited at 90% maximum due to the power loss of fly-wheeling diode is proportioned with its junction voltage drop that dominate the total power loss. In order to overcome limitations in power loss and heat, the prior art has been devised the synchronous rectification technology.
FIG. 1a shows the synchronous rectification buck converter named as SR-Buck converter below. The SR-Buck converter has been derived from conventional buck converter by using the semiconductor switch such as MOSFET to replace fly-wheeling diode. The forward switch SFW is drive by a PWM signal for turned ON and turned OFF periodically to transfer the energy from input voltage source VI through inductor LO to smooth capacitor CO and load RL. The fly-wheeling switch SFL is also drive by a PWM signal that complementary with the drive signal of forward switch SFW for turned ON and OFF periodically to discharge the stored energy of inductor LO to the smooth capacitor CO and load RL. A small period of dead time between the turn ON time of forward switch SFW and fly-wheeling switch SFL is required to prevent the damage from short through behavior between SFW and SFL. FIG. 1b shows the timing diagram of the circuit of FIG. 1a. 
SR-Buck converter reduces the voltage drop of diode and its power loss allows the conversion efficiency is greater then 90% that the power handling of converter is improved. However, the optimal gate drive waveform is generated from high circuit complexity controller to keep the turned ON time of forward switch SFW and fly-wheeling switch SFL never overleap but as small as possible to obtain highest efficiency that the component selection is difficult to meet high reliability and cost effective requirement.
FIG. 2a shows the resonant reset forward converter with secondary passive drive synchronous rectifier circuit named as SR-Forward converter below. The secondary synchronous rectifier circuit has been derived from conventional rectifier by using semiconductor switch such as MOSFET to replace semiconductor diode each leg. The forward switch SFW is drive by secondary winding WS to selects connecting the output inductor to secondary winding transfer energy to smooth capacitor CO and load RL when the primary switch SP is turned ON to connect primary winding WP with input voltage source VI. The fly-wheeling switch SFL may selects connecting the output inductor to ground during the reset time of transformer when the primary switch SP is turned OFF. The gate drive signal of primary switch SP is supplied from a simple PWM controller and the gate driving signals of forward switch SP and fly-wheeling switch SFL are supplied from transformer secondary directly. FIG. 2b shows the timing diagram of the circuit of FIG. 2a. 
The SR-Forward converter reduce the power loss with minimum circuit complexity of controller, it take advantage from synchronous rectifier circuit to reach high efficiency just like most topologies. However, the fly-wheeling switch SFL will be turned OFF due to the reset voltage drop to zero before primary switch SP turned ON, it cause extra power loss due to low speed high voltage drop body diode conduct the load current limit the input voltage range for efficient operation.
What is the best power conversion technology? Higher efficiency, less circuit complexity of converter and wide range operation are the most important demands of power conversion technology.